pH-Controlled Electrodeposition of Diameter-Modulated Co Nanowires: Crystal Texture and Magnetic Properties

Abstract

Diameter-modulated (DM) and homogeneous-diameter cobalt nanowires (NWs) were electrodeposited into the ordered pores of anodic aluminum oxide templates from sulphate solutions with pH=4.0 and pH=6.5. The role played by these pH conditions on the electrochemical growth, crystalline structure and magnetic properties of such nanowire arrays has been investigated. Microstructural characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows that DM Co NWs electrodeposited in pH = 6.5 exhibit hcp highly textured polycrystalline lattice with preferred growth orientation along the [100] direction parallel to the nanowires’ axis. However, those NWs electrodeposited in pH =4.0 exhibit a hcp polycrystalline lattice structure without any preferential crystalline texture. It is concluded that this crystalline growth is a consequence of the co-evolution of hydrogen with metallic cobalt, whose growth rate decreases at low pH and higher hydrogen concentration of ions, due to limition of growth sites, which are blocked by gas bubbles. XRD and TEM analysis inform that the diameter modulation did not show a clear effect on the crystallographic texture of nanowires. Magnetization loops, measured by vibrating sample magnetometry (VSM) under parallel and perpendicular magnetic field configurations, show reduced irreversibilities and reveal a pH dependent magnetic response. For pH = 6.5, coercivity and squareness (in parallel and perpendicular configurations) decrease from 164 Oe and 5.1 % to 152 Oe and 2.6 %, respectively, while for pH = 4, they increase from 182 Oe and 3 % to 203 Oe and 4 %, respectively. For pH = 6.5, DM nanowires show a (100) texture, aligning the c-axis perpendicular to the nanowires and yielding a competition between shape and magnetocrystalline anisotropies, whereas for pH = 4.0, polycrystalline nanowires have no defined magnetocrystalline easy axis. For both pH, a significant magnetostatic energy term is concluded arising from the shape anisotropy of individual nanowires plus significant interactions among neighboring DM nanowires.